Creating a set of spotlights on one dimmer switch
You should copy them with Alt+D, which would mean they are linked duplicates. Then changing the emission value on one would change all them at the same time. This is the solution as long as it's okay for all of them to have exactly the same properties. 

If you need them to differ on some properties, you would need to select them all, change the emit value and then right click on the emit value and select "Copy to Selected" if you just want to update one value on all the selected lamps.